The present disclosure relates to an image pickup element having a photoelectric conversion section including a chalcopyrite-based compound, and to an image pickup device including the image pickup element.
For a solid-state image pickup device such as a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor, a reduction in pixel size is desired, as the number of pixels increases. Meanwhile, there is a demand for improvement in moving image properties through high-speed image pickup. In such reduction in pixel size and high-speed image pickup, the number of photons entering a unit pixel (a solid-state image pickup element) decreases, which reduces sensitivity (a signal-to-noise (S/N) ratio). Further, for example, an image-taking function in a dark place may be demanded of a monitoring camera. Therefore, an image pickup device that achieves high sensitivity, in addition to an increase in the number of pixels and a reduction in pixel size, is desired.
For example, in photoelectric conversion devices (solid-state image pickup devices) of Japanese Unexamined Patent Application Publication No. 2007-123721, and International Publication Nos. WO 2008/093834, and WO 2009/078299, sensitivity is increased using a chalcopyrite-based compound semiconductor having a high optical-absorption coefficient, for a photoelectric conversion film. These photoelectric conversion devices have a configuration in which a photoelectric conversion film is interposed between an n-type semiconductor disposed on incident-light side, and a lower electrode disposed on a side opposite to the incident-light side. These photoelectric conversion devices have such a structure that, of an electron-hole pair generated by photoelectric conversion, the electron is discharged to the n-type semiconductor, while the positive hole is transferred by the lower electrode and then read out in a silicon circuit.
In the photoelectric conversion devices having such a structure, electric charge (the positive hole) generated by the photoelectric conversion during exposure time is stored in a capacitor through the lower electrode. Therefore, removal of kTC noise (thermal noise) in reset operation is difficult, which may degrade image quality. To address this situation, for example, in a solid-state image pickup device of Japanese Unexamined Patent Application Publication No. 2012-4443 (JP2012-4443A), a photoelectric conversion film made of a chalcopyrite-based compound semiconductor is provided on a Si substrate where an n-type semiconductor region is formed. Further, an upper electrode is disposed on an incident surface side of this photoelectric conversion film. In this solid-state image pickup device, of an electron-hole pair generated by photoelectric conversion, the positive hole is discharged to the upper electrode, while the electron is stored in a junction capacitor through the depleted n-type semiconductor. As a result, the kTC noise is removed.